U.S. patent number 6,725,093 [Application Number 09/831,100] was granted by the patent office on 2004-04-20 for regulation of excitable tissue control of the heart based on physiological input.
This patent grant is currently assigned to Impulse Dynamics N.V.. Invention is credited to Shlomo Ben-Haim, Nissim Darvish, Itzhak Shemer, Yehuda Snir.
United States Patent |
6,725,093 |
Ben-Haim , et al. |
April 20, 2004 |
Regulation of excitable tissue control of the heart based on
physiological input
Abstract
A method and apparatus (30) for modifying contractility of the
heart of a patient. The method includes receiving signals from a
sensor (22, 23, 46) coupled to the body of the patient indicative
of physiological activity. The signals are analyzed to derive a
measure of the physiological activity, and excitable tissue control
(ETC) stimulation is applied to the heart (20) so as to enhance
contractility of the heart muscle responsive to the measure.
Inventors: |
Ben-Haim; Shlomo (Haifa,
IL), Darvish; Nissim (Haifa, IL), Shemer;
Itzhak (Haifa, IL), Snir; Yehuda (Yokneam Illit,
IL) |
Assignee: |
Impulse Dynamics N.V. (Curacao,
NL)
|
Family
ID: |
32073945 |
Appl.
No.: |
09/831,100 |
Filed: |
September 10, 2001 |
PCT
Filed: |
November 04, 1999 |
PCT No.: |
PCT/IL99/00594 |
PCT
Pub. No.: |
WO00/27476 |
PCT
Pub. Date: |
May 18, 2000 |
Foreign Application Priority Data
Current U.S.
Class: |
607/9; 607/19;
607/2 |
Current CPC
Class: |
A61N
1/3627 (20130101); A61N 1/36585 (20130101); A61N
1/36542 (20130101) |
Current International
Class: |
A61N
1/365 (20060101); A61N 1/362 (20060101); A61N
001/362 () |
Field of
Search: |
;607/4-6,9,19 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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|
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|
WO 97/25098 |
|
Jul 1997 |
|
WO |
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WO 98/10830 |
|
Mar 1998 |
|
WO |
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WO 98/10831 |
|
Mar 1998 |
|
WO |
|
WO 98/10832 |
|
Mar 1998 |
|
WO |
|
WO 00/04947 |
|
Feb 2000 |
|
WO |
|
Other References
A H. Foster et al., "Acute Hemodyamic Effects of
Atrio-Biventricular Pacing in Humans", 1995, The Society of
Thoracic Surgeons vol. 59, pp. 294-299. .
S. Cazeau et al., "Multisite Pacing for End-Stage Heart Failure:
Early Experience", Pacing and Clinical Electrophysiology vol. 19,
Nov. 1996, Part II, pp. 1748-1757. .
Yu et al., "Does Biventricular Pacing Provide Better Cardiac
Function than Univentricular Pacing in Normal Dogs?" Abstract,
Heart Failure Society Abstracts-on-Disk.RTM., Sep. 13-16, 1998,
Boca Raton, Florida, one page. .
A. Auricchio et al., "Acute Pacing of the Left Ventricle is
Associated with Largest Hemodynamic Improvement in PATH-CHF Heart
Failure Patients", Abstract, Heart Failure Society
Abstracts-on-Disk.RTM., Sep. 13-16, 1998, Boca Raton, Florida, one
page. .
C. Leclercq et al., "Comparative Effects of Permanent Biventricular
Pacing in Class III and Class IV Patients", Pacing and Clinical
Electrophysiology, Apr. 1998, vol. 21, No. 4, Part II, p. 911.
.
P. F. Bakker et al., "Beneficial Effects of Biventricular Pacing of
Congestive Heart Failure", PACE, vol. 17, Apr. 1994, Part II, one
page. .
P. F. Bakker et al., Biventricular Pacing Improves Functional
Capacity in Patients with End-Stage Congestive Heart Failure,PACE,
Apr. 1995, Part II, one page. .
H. Antoni et al., "Polarization Effects of Sinusoidal 50 Hz
Alternating Current on Membrane Potential of Mammalian Cardiac
Fibres", Pflugers Arch. 314, pp. 274-291, 1970. .
Guidant Product Catalogue, 2001 2 pages. (as printed)
(http://www.guidant.com/glossary.htm.)..
|
Primary Examiner: Layno; Carl
Attorney, Agent or Firm: Reed Smith LLP Dippert; William
H.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This patent application claims the benefit of U.S. Provisional
Patent Application 60/107,479, filed Nov. 6, 1998 which is assigned
to the assignee of the present patent application and is
incorporated herein by reference.
Claims
What is claimed is:
1. A method for modifying contractility of the heart of a patient,
comprising: receiving signals from a sensor coupled to the body of
the patient indicative of physiological activity; analyzing the
signals to derive a measure of the physiological activity; and
applying excitable tissue control (ETC) stimulation to the heart so
as to enhance contractility of the heart muscle responsive to the
measure, wherein applying the stimulation comprises assessing the
measure so as to determine in which of a plurality of predetermined
ranges the measure falls, and varying the application of the ETC
stimulation dependent on the range, wherein assessing the measure
comprises associating one or more of the ranges with respective
types of physical activity undertaken by the patient, and wherein
varying the application comprises adapting the stimulation to a
demand level associated with the physical activity, and wherein
adapting the stimulation comprises increasing the stimulation
responsive to an increasing demand level.
2. A method for modifying contractility of the heart of a patient,
comprising: receiving signals from a sensor coupled to the body of
the patient indicative of physiological activity; analyzing the
signals to derive a measure of the physiological activity; and
applying excitable tissue control (ETC) stimulation to the heart so
as to enhance contractility of the heart muscle responsive to the
measure, wherein applying the stimulation comprises assessing the
measure so as to determine in which of a plurality of predetermined
ranges the measure falls, and varying the application of the ETC
stimulation dependent on the range, wherein assessing the measure
comprises associating one or more of the ranges with respective
types of physical activity undertaken by the patient, and wherein
varying the application comprises adapting the stimulation to a
demand level associated with the physical activity, and wherein
adapting the stimulation comprises decreasing the stimulation
responsive to an increasing demand level.
3. A method for modifying contractility of the heart of a patient,
comprising: receiving signals from a sensor coupled to the body of
the patient indicative of physiological activity; analyzing the
signals to derive a measure of the physiological activity; and
applying excitable tissue control (ETC) stimulation to the heart so
as to enhance contractility of the heart muscle responsive to the
measure, wherein analyzing the signals comprises deriving a measure
of hemodynamic stress.
4. Apparatus for stimulating cardiac tissue in the body of a
patient, comprising: at least one sensor, coupled to the body which
generates signals indicative of physiological activity; one or more
stimulation electrodes, which are placed in contact with the heart;
and an electrical control unit, which receives and analyzes the
signals from the sensor so as to derive a measure of the
physiological activity and which applies an excitable tissue
control (ETC) signals to the stimulation electrodes so as to
enhance contractility of the heart muscle responsive to the
measure, wherein the control unit assigns the measure to one of a
plurality of predetermined ranges and varies the application of the
ETC signals dependent on the range, wherein the control unit holds
off application of the signal when the measure is outside a range
between predetermined lower and upper thresholds.
5. Apparatus for stimulating cardiac tissue in the body of a
patient, comprising: at least one sensor, coupled to the body which
generates signals indicative of physiological activity; one or more
stimulation electrodes, which are placed in contact with the heart;
and an electrical control unit, which receives and analyzes the
signals from the sensor so as to derive a measure of the
physiological activity and which applies an excitable tissue
control (ETC) signals to the stimulation electrodes so as to
enhance contractility of the heart muscle responsive to the
measure, wherein the control unit assigns the measure to one of a
plurality of predetermined ranges and varies the application of the
ETC signals dependent on the range, wherein one or more of the
ranges are associated with respective types of physical activity
undertaken by the patient, and the control unit adapts the
stimulation to a demand level associated with the physical
activity, wherein the control unit increases the stimulation
responsive to an increasing demand level.
6. Apparatus for stimulating cardiac tissue in the body of a
patient, comprising: at least one sensor, coupled to the body which
generates signals indicative of physiological activity; one or more
stimulation electrodes, which are placed in contact with the heart;
and an electrical control unit, which receives and analyzes the
signals from the sensor so as to derive a measure of the
physiological activity and which applies an excitable tissue
control (ETC) signals to the stimulation electrodes so as to
enhance contractility of the heart muscle responsive to the
measure, wherein the control unit assigns the measure to one of a
plurality of predetermined ranges and varies the application of the
ETC signals dependent on the range, wherein one or more of the
ranges are associated with respective types of physical activity
undertaken by the patient, and the control unit adapts the
stimulation to a demand level associated with the physical
activity, and wherein the control unit decreases the stimulation
responsive to an increasing demand level.
7. Apparatus for stimulating cardiac tissue in the body of a
patient, comprising: at least one sensor, coupled to the body which
generates signals indicative of physiological activity; one or more
stimulation electrodes, which are placed in contact with the heart;
and an electrical control unit, which receives and analyzes the
signals from the sensor so as to derive a measure of the
physiological activity and which applies an excitable tissue
control (ETC) signals to the stimulation electrodes so as to
enhance contractility of the heart muscle responsive to the
measure, wherein the measure comprises a measure of hemodynamic
stress.
8. A method for cardiac rehabilitation therapy, comprising:
receiving signals from a sensor coupled to the body of a patient
indicative of physiological activity; analyzing the signals to
derive a measure of the physiological activity, the measure having
a range of values; associating the values of the measure with
levels of physical activity undertaken by the patient; and applying
electrical stimulation to the heart so as to induce muscular
exertion thereof responsive to the level, such that over at least a
part of the range, the stimulation is adjusted to reduce the
muscular exertion of the heart responsive to an increase in the
level of activity, wherein applying the stimulation comprises
inducing exertion of the heart while the patient is at rest and
reducing the exertion when the patient is active.
9. A method for cardiac rehabilitation therapy, comprising:
receiving signals from a sensor coupled to the body of a patient
indicative of physiological activity; analyzing the signals to
derive a measure of the physiological activity, the measure having
a range of values; associating the values of the measure with
levels of physical activity undertaken by the patient; and applying
electrical stimulation to the heart so as to induce muscular
exertion thereof responsive to the level, such that over at least a
part of the range, the stimulation is adjusted to reduce the
muscular exertion of the heart responsive to an increase in the
level of activity, wherein applying the stimulation comprises
applying excitable tissue control (ETC) stimulation to the heart so
as to enhance contractility of the heart muscle.
Description
FIELD OF THE INVENTION
The present invention relates generally to invasive devices and
methods for treatment of the heart, and specifically to devices and
methods for electrical stimulation of the heart muscle.
BACKGROUND OF THE INVENTION
Demand-responsive pacemakers are known in the art. Such devices
provide pulses to pace the heart of a patient at a variable rate,
dependent on signals received from the body of the patient.
PCT patent application PCT/IL97/00012, published as WO 97/25098, to
Ben-Haim et al., whose disclosure is incorporated herein by
reference, describes methods for modifying the force of contraction
of at least a portion of a heart chamber by applying a
non-excitatory electric field to the heart at a delay after
electrical activation of the portion. The non-excitatory field is
such as does not induce activation potentials in cardiac muscle
cells, but rather modifies the cells' response to subsequent
activation. In the context of the present patent application, the
use of such a non-excitatory field is referred to as Excitable
Tissue Control (ETC). The non-excitatory field may be applied in
combination with a pacemaker or defibrillator, which applies an
excitatory signal (i.e., pacing or defibrillation pulses) to the
heart muscle.
SUMMARY OF THE INVENTION
It is an object of some aspects of the present invention to provide
improved methods and apparatus for Excitable Tissue Control (ETC)
of the heart so as to enhance hemodynamic performance thereof.
In preferred embodiments of the present invention, a cardiac
stimulation device comprises one or more ETC electrodes, at least
one sensor for sensing physiological activity, and electronic
control circuitry, coupled to the ETC electrodes and sensor. The
ETC electrodes and, preferably, the sensor are placed at selected
sites in the heart of a patient. Alternatively, the sensor may be
placed elsewhere inside or on a surface of the patient's body. The
circuitry receives signals from the sensor and analyzes the signals
to determine a measure of the physiological activity. Responsive to
the measure, the circuitry drives the stimulation electrodes to
provide ETC stimulation so as to enhance contractility of the heart
muscle. Preferably, the circuitry assesses the measure so as to
determine in which of a plurality of predetermined ranges the
measure falls, and controls intensity of the ETC stimulation
dependent on the range.
In some preferred embodiments of the present invention, the sensor
comprises a heart rate sensor, and the circuitry assigns the
measured heart rate to one of several heart rate ranges.
Preferably, lower and upper heart rate thresholds are assigned, and
the circuitry holds off the ETC stimulation when the heart rate is
in a range below the lower threshold or above the upper one. When
the heart rate is between the lower and upper thresholds, the
circuitry preferably applies the ETC stimulation, while adjusting
the intensity of the stimulation according to a predetermined
function of the measured heart rate. In this manner, safety of the
ETC stimulation is improved, and the intensity of the stimulation
is adjusted so as to provide enhancement of the contractility, and
hence of hemodynamic performance, when and as needed by the
patient. Controlling the ETC stimulation by this method also
reduces power consumption by the device and thus increases battery
lifetime when the device is implanted in the patient's body.
In preferred embodiments of the present invention, the circuitry
times the application of ETC stimulation so that the stimulation is
applied at a fixed time, preferably with a predetermined delay,
relative to electrical activation of the heart. The electrical
activation is typically due to pacing pulses applied to the heart,
but may also be due to the normal sinus rhythm, which is preferably
detected by the sensor. In some preferred embodiments of the
present invention, the circuitry controls the intensity of the ETC
stimulation by counting heart beats in sequence and applying the
stimulation only at certain of the beats in the sequence. In the
context of the present patent application and in the claims, this
mode of intensity control is referred to as duty cycle modulation.
The inventors have found that ETC has a cumulative effect on heart
muscle contractility over a period of many heart beats, and
therefore it is believed that duty cycle modulation is a simple and
effective means of controlling the intensity of the
stimulation.
PCT patent application PCT/IL97/00235, and the corresponding U.S.
patent application Ser. No. 09/254,900, which are assigned to the
assignee of the present patent application and whose disclosures
are incorporated herein by reference, describe a cardiac output
controller using ETC stimulation. Control circuitry receives
signals from one or more sensors, indicative of the heart's
activity, and responsive thereto, drives the stimulation electrodes
to provide the ETC stimulation to the heart. The effect of the
controller on cardiac output is regulated by changing the timing of
the non-excitatory stimulation pulse relative to the heart's
activity, preferably relative to the heart's electrical activity of
ECG signals received by the sensor (which comprises a sensing
electrode). Alternatively or additionally, the controller changes
other pulses characteristics, such as the voltage, current,
duration, polarity, shape and frequency of the waveform, and delay
of the ETC pulse relative to a pacing pulse or to sensing of an
activation potential in the heart. The sensors may also include
flow rate sensors, pressure sensors, temperature sensors, oxygen
sensors, and other types of sensors known in the art, so as to
provide additional signals indicative of hemodynamic conditions,
such as cardiac output, blood pressure or blood oxygenation.
PCT patent application PCT/IL97/00236, and the corresponding U.S.
patent application Ser. No. 09/254,900, which are assigned to the
assignee of the present patent application and whose disclosures
are incorporated herein by reference, describe a pacemaker that
gives cardiac output enhancement. This pacemaker applies both
excitatory (pacing) and non-excitatory (ETC) electrical stimulation
pulses to the heart. By applying non-excitatory pulses of suitable
strength, appropriately timed with respect to the heart's
electrical activation, the contraction of selected segments of the
heart muscle can be increased or decreased, thus increasing or
decreasing the stroke volume of the heart.
Further aspects of the ETC are described in U.S. patent application
Ser. No. 09/101,723, which is similarly assigned to the assignee of
the present patent application and whose disclosure is incorporated
herein by reference. The application corresponds to the
above-mentioned PCT patent application PCT/IL97/00012.
Israel patent application 125,424, which is assigned to the
assignee of the present patent application and whose disclosure is
incorporated herein by reference, describes a cardiac pacemaker
that applies an extended pacing signal to the heart, thus enabling
simultaneous pacing and ETC stimulation of the heart. The signal
typically comprises a pacing pulse or a periodic waveform,
preferably made up of a train of pulses, having an overall duration
substantially longer than a pulse duration required for pacing the
heart. The pacemaker is controlled to selectively apply either the
extended pacing signals or ordinary, standard pacing signals, as
indicated by the patient's transient and long-term hemodynamic
needs.
Although preferred embodiments of the present invention are
described in terms of certain specific types of sensors, typically
sensing electrodes, and methods of applying and controlling the
intensity of ETC stimulations, such as duty cycle modulation, the
scope of the present invention is in no way limited to these
modalities. It will be understood that the principles of the
present invention may be applied using any other suitable types of
sensors, ETC modalities and methods of controlling ETC stimulation
intensity, including (but not limited to) those described in the
above-mentioned PCT and Israel patent applications.
There is therefore provided, in accordance with a preferred
embodiment of the present invention, a method for modifying
contractility of the heart of a patient, including: receiving
signals from a sensor coupled to the body of the patient indicative
of physiological activity; analyzing the signals to derive a
measure of the physiological activity; applying excitable tissue
control (ETC) stimulation to the heart so as to enhance
contractility of the heart muscle responsive to the measure.
Preferably, applying the stimulation includes applying electrical
signals to stimulate the heart and controlling intensity of the
signals responsive to the measure, wherein controlling the
intensity includes regulating a duty cycle of the signals relative
to a beat rate of the heart.
Further preferably, applying the stimulation includes assessing the
measure so as to determine in which of a plurality of predetermined
ranges the measure falls, and varying the application of the ETC
stimulation dependent on the range. Preferably, assessing the
measure includes setting upper and lower thresholds with respect to
the measure, and varying the application of the stimulation
includes holding off the stimulation when the measure is outside a
range between the thresholds.
Preferably, assessing the measure includes associating one or more
of the ranges with respective types of physical activity undertaken
by the patient, and wherein varying the application includes
adapting the stimulation to a demand level associated with the
physical activity. Most preferably, adapting the stimulation
includes increasing the stimulation responsive to an increasing
demand level or alternatively, decreasing the stimulation
responsive to an increasing demand level.
In a preferred embodiment, analyzing the signals includes deriving
a measure of hemodynamic stress.
Preferably, the measure includes a heart rate.
In another preferred embodiment, receiving the signals includes
receiving a signal responsive to motion of the patient.
There is also provided, in accordance with a preferred embodiment
of the present invention, apparatus for stimulating cardiac tissue
in the body of a patient, including: at least one sensor, coupled
to the body which generates signals indicative of physiological
activity; one or more stimulation electrodes, which are placed in
contact with the heart; and an electrical control unit, which
receives and analyzes the signals from the sensor so as to derive a
measure of the physiological activity and which applies an
excitable tissue control (ETC) signals to the stimulation
electrodes so as to enhance contractility of the heart muscle
responsive to the measure.
Preferably, the at least one sensor includes an accelerometer.
Alternatively or additionally, the at least one sensor includes a
sensing electrode, wherein the sensing electrode preferably
includes one of the stimulation electrodes.
There is further provided, in accordance with a preferred
embodiment of the present invention, a method for cardiac
rehabilitation therapy including: receiving signals from a sensor
coupled to the body of a patient indicative of physiological
activity; analyzing the signals to derive a measure of the
physiological activity, the measure having a range of values;
associating the values of the measure with levels of physical
activity undertaken by the patient; applying electrical stimulation
to the heart so as to induce muscular exertion thereof responsive
to the level, such that over at least a part of the range, the
stimulation is adjusted to reduce the muscular exertion of the
heart responsive to an increase in the level of activity.
Preferably, applying the stimulation includes inducing exertion of
the heart while the patient is at rest and reducing the exertion
when the patient is active. Further preferably, includes applying
excitable tissue control (ETC) stimulation to the heart so as to
enhance contractility of the heart muscle.
The present invention will be more fully understood from the
following detailed description of the preferred embodiments
thereof, taken together with the drawings in which:
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic illustration of an Excitable Tissue Control
(ETC) device applied to a heart, in accordance with a preferred
embodiment of the present invention;
FIG. 2A is a graph that schematically shows pacing and ETC signals
applied using the device of FIG. 1, in accordance with a preferred
embodiment of the present invention;
FIG. 2B is a graph showing a detail of the signals of FIG. 2A;
FIG. 3 is a graph showing a schematic histogram of typical heart
rate measurements made over the course of a day; and
FIGS. 4-7 are graphs that schematically illustrate functions used
in controlling ETC stimulation, in accordance with preferred
embodiments of the present invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
Reference is now made to FIG. 1, which is a schematic illustration
showing an ETC stimulation device 30, which is applied to pace and
stimulate a heart 20, in accordance with a preferred embodiment of
the present invention. Details of the design and construction of
devices such as device 30 are provided in the above-mentioned U.S.
and PCT patent applications, as well as in the above-mentioned U.S.
Provisional Patent Application 60/104,479.
Device 30 comprises pacing circuits 32 and ETC circuits 36, which
are respectively coupled to drive one or more pacing electrodes 34
and one or more ETC electrodes 38. As shown in FIG. 1, pacing
electrodes 34 are typically applied in right atrium 22 and
optionally right ventricle 23, and ETC electrodes 38 are applied to
left ventricle 24, most preferably epicardially or intravenously to
the free wall of the left ventricle. Greater numbers of electrodes
and different electrode placements are also possible.
Sensing circuits 40 receive electrogram signals from heart 20,
which signals are preferably provided by the pacing and/or ETC
electrodes (although separate sensing electrodes can also be used
for this purpose). Control circuitry 44, preferably comprising a
microprocessor and a memory, for storing programs and data.
Circuitry 44 receives the signals processed by sensing circuits 40
and, based on the signals, derives the heart rate and optionally
other parameters relating to cardiac function, as well. Preferably,
an accelerometer or other sensor 46 provides signals to circuitry
44 responsive to motion, i.e., physical activity of the patient.
Other sensors of any suitable type known in the art may also be
used. Responsive to the signals from sensing circuits 40 and sensor
46, circuitry 44 controls the application of pacing and ETC
stimulation to the heart, as described hereinbelow.
FIG. 2A is a graph that schematically illustrates a sequence of
pacing pulses 50 and ETC signals 52 applied by device 20, in
accordance with a preferred embodiment of the present invention.
The horizontal (time) axis is not drawn to scale. Over a time
period 54 of about 10 sec, pacing pulses 50 are applied regularly,
whereas ETC signals 52 are applied only following the first three
pacing pulses during the period. This pattern of three ETC signals
on, followed by seven off, is repeated in subsequent time periods,
and is referred to herein as 30% duty cycle operation. The duty
cycle may be varied, by decreasing or increasing the number of ETC
signals in each period, between 0 and 100%. The greater the duty
cycle, the more intense will be the typical effect of the ETC
stimulation on cardiac contractility. Controller 44 thus instructs
ETC circuit 36 to vary the duty cycle of the ETC signals so to
control the ETC intensity, responsive to the sensor signals
received from sensing circuit 40.
FIG. 2B is a graph that schematically shows a detail of pacing
pulse 50 and ETC signal 52 shown in FIG. 2A. The horizontal (time)
axis is not drawn to scale. Pacing pulse 50 generally comprises a
standard pacing pulse of suitable amplitude, having a duration of
about 2 ms or less, as is well known in the art. ETC signal 52 is
preferably delayed by a time T relative to the pacing. T is chosen
so that the ETC signal begins after the area of heart 20 to which
electrode 38 is applied has already been electrically activated,
i.e., while the heart tissue is refractory. Signal 52 preferably
comprises a train of pulses 56, most preferably bipolar pulses,
typically having an amplitude, a pulse period .tau., and a total
duration T.sub.p that are substantially greater than the amplitude
and duration of pacing pulse 50. Further details regarding
characteristics of ETC signal 52, as well as a wide variety of
other waveforms that may be used for ETC in place of signal 52, are
described in the above-mentioned patent applications.
FIG. 3 is a schematic histogram illustrating a distribution 60 of
heart rates that would typically be measured in monitoring cardiac
activity of a patient over the course of a day. The distribution
could be generated by spontaneous sinus rhythm of the heart or by a
demand-responsive pacemaker, as is known in the art. Distribution
60 is shown here as an aid in understanding the methods of ETC
stimulation described hereinbelow, and is not meant to be an
accurate representation of measurements made on a particular
patient.
Distribution 60 generally comprises three major lobes, disregarding
measurement artifacts falling outside the limits of the
distribution. Generally, while the patient is asleep, his or her
heart rate maintains a slow, steady rate falling within a lower
lobe 62. During low-intensity waking activities, the heart rate
rises to within a middle lobe 64. During intense activity or
exertion, the heart rate is generally in upper lobe 66.
Abnormalities such as arrhythmias not controlled by pacing may
cause readings of fast heart rates while at rest or abnormally slow
heart rates during waking activities, which fall outside their
proper lobes. Such abnormalities are preferably revealed by
comparing readings from accelerometer 46 (FIG. 1) to the heart rate
measurements.
FIG. 4 is a graph that schematically illustrates a function 70 used
in controlling ETC stimulation responsive to heart rate
measurements, in accordance with a preferred embodiment of the
present invention. The function gives the ETC duty cycle as a
function of measured heart rate and is applied by device 30, or by
another suitable, similar device, in generating a sequence of ETC
signals such as that shown in FIG. 2A. As described hereinabove,
the duty cycle of the ETC stimulation is varied between minimum and
maximum values, preferably 0 and 100%, by increasing or decreasing
the number of signals 52 applied by ETC circuit 36 during time
period 54.
The principle exemplified by function 70 is that ETC stimulation is
to be applied with greater intensity, i.e., at higher duty cycle,
in proportion to the body's demand for cardiac output. Thus, below
a minimum heart rate value, preferably around 60 bpm, corresponding
generally to sleep and resting states of the patient, the duty
cycle is held at its minimum value. No ETC stimulation is applied,
since there is no need to boost the patient's cardiac output. From
the minimum rate up to a middle value, preferably about 90 bpm, in
a range 72 corresponding generally to low-intensity waking
activities, the duty cycle increases gradually as a function of
heart rate, reaching the maximum value at a knee 76. For
high-demand activity, with heart rate in a range 74, ETC intensity
is maintained at its maximum value. Above a maximum point 78,
preferably at about 120 bpm, the ETC stimulation is cut off, since
there may be a danger of placing excessive strain on the heart and,
furthermore, the high heart rate may be due to tachycardia or
incorrect measurements. Preferably, control circuitry 44 is
programmable, so that the set points of function 70 and the maximum
and minimum values of the duty cycle can be adjusted for the needs
of the particular patient.
FIG. 5 is a graph that schematically illustrates another function
80 used to control ETC stimulation responsive to heart rate
measurements, in accordance with an alternative preferred
embodiment of the present invention. Function 80 is similar in
principle to function 70, as described hereinabove, but differs in
that in applying the function, control circuitry 44 takes into
account not only the instantaneous measured heart rate, but also
the variation of the heart rate over time. Thus, when the patient's
heart rate begins to increase from the minimum toward the middle of
the heart rate range, as occurs when the patient begins some
strenuous activity, circuitry 44 boosts the ETC duty cycle rapidly,
as indicated by a curve 82 in function 80. Because of the gradual
onset of enhanced contractility when ETC stimulation is initiated,
the heart's stroke volume will likewise increase gradually in
parallel with the increased heart rate. Optionally, ETC stimulation
intensity may be decreased more rapidly below point 76 when the
heart rate is decreasing, as indicated by a curve 84. Preferably,
the rates of increase and decrease of the ETC signal intensity, as
functions of increasing or decreasing heart rate, are programmable,
along with other signal parameters.
It is also noted that function 80 includes a gradual cutoff of ETC
duty cycle over a range 86 above maximum point 78, rather than a
sharp cutoff as in function 70.
Heart rate variation may also, in itself, be used as an indicator
for controlling the intensity of ETC stimulation. It is known in
the art that while the body is at rest, the heart rate tends to
have a high degree of variability, i.e., there are relatively large
changes in the instantaneous heart rate from beat to beat. On the
other hand, during exertion, when the heart is under stress, the
heart rate becomes nearly constant. Thus, in a preferred embodiment
of the present invention not shown in the figures, the ETC duty
cycle is adjusted so as to provide ETC intensity that increases as
an inverse function of heart rate variability. Most preferably,
such heart rate variability assessment is used in conjunction with
other parameters, such as the heart rate itself and signals from
accelerometer 46, in setting the ETC intensity level. Changes in
blood pH and in temperature may also be measured and used by
circuitry 44 in conjunction with the heart rate in assessing
physiological stress, as is known in the art, and thus determining
when an increase in ETC intensity will be needed.
Other parameters relating to cardiac stress may also be sensed by
sensor 42 (assuming the sensor is of an appropriate type), and used
by control circuitry 44 in determining the intensity of ETC
stimulation to be applied. In particular, circuitry 44 may receive
or derive from measured parameters an indication of cardiac
ischemia, and responsive to the indication may terminate or reduce
the intensity of the ETC stimulation in order to prevent infarction
or undue strain on the heart muscle. Such indications of ischemia
may include, for example, a shift of the ST segment in ECG or
electrogram signals or a drop in blood oxygen saturation measured
in the coronary sinus.
FIG. 6 is a graph that schematically illustrates a set 90 of
functions 92, 94 and 96 used in controlling the intensity of ETC
stimulation at various levels of patient activity, in accordance
with a preferred embodiment of the present invention. Function 92
corresponds to a low activity level, such as rest, while functions
94 and 96 correspond to higher levels of activity and stress. The
functions are generally similar in shape and implementation to
functions 70 and 80, as described hereinabove, but are adjusted for
the patient's physical activity level, preferably as measured by
accelerometer 46 or other means. Generally, the functions are
chosen such that the greater the patient's activity or exertion,
the higher will be the intensity of ETC stimulation and the wider
will be the range between points 76 and 78 over which the maximal
stimulation is applied.
Using set 90, rather than a single function, is helpful in that it
enables the ETC stimulation to be applied most strongly, per
function 94 or 96, to meet the real need for increased cardiac
output that is incurred in physical activity. The strength of
stimulation is suppressed, per function 92, when physical activity
is low, since under such circumstances it would appear that the
increased heart rate is due to abnormalities or disease factors.
Set 90 is thus useful in reducing power consumption by device 30
and minimizing unnecessary strain on the patient's heart.
FIG. 7 is a graph that schematically illustrates another function
100, which is used, in accordance with a preferred embodiment of
the present invention, to control ETC stimulation in patients
suffering from serious heart disease, such as congestive heart
failure (CHF). For such a patient, whose heart has been severely
weakened, device 30 helps to condition and strengthen the
contractility of the heart muscle, in order to alleviate the
cardiac and systemic symptoms of the disease. Therefore, the
objective of function 100 is not so much to provide additional
cardiac output in response to physiological demand, as in the case
of functions 70, 80, 92, 94 and 96, but rather to provide
stimulation that will exercise the heart without causing
intolerable stress to the heart muscle. Thus, when the patient's
heart rate is in a low range 102 between minimal and middle values,
ETC stimulation is applied at a maximal intensity, although
preferably at a duty cycle that is substantially less than 100%.
Above the middle value, the stimulation intensity drops off over a
range 104, in response to the strain imposed by the increasing
heart rate.
Preferably, circuitry 44 is programmed to operate in accordance
with function 100 during an initial conditioning period after
beginning treatment with device 30. Once the patient's recovery has
progressed and the heart muscle has been strengthened sufficiently,
circuit 44 is reprogrammed so that it operates in accordance with a
function such as function 70 or 80 or set 90.
Although in the preferred embodiments described above, the ETC
intensity is controlled by varying the duty cycle of the ETC
signals relative to the heart beat, it will be appreciated that
many other methods can be used to control the ETC intensity, and
all of these methods are within the scope of the present invention.
In addition to controlling the duty cycle of ETC signals 52, other
signal parameters may be controlled, including the signal
amplitude, duration, delay, waveform shape and frequency, polarity,
and DC offset. When multiple ETC electrodes 38 are used, the
signals may also be applied to greater or lesser numbers of the
electrodes and/or to electrodes located in different areas of the
heart, dependent on the measurements of cardiac activity and
stress.
It will thus be appreciated that the preferred embodiments
described above are cited by way of example, and the full scope of
the invention is limited only by the claims.
* * * * *
References